1. Field of Invention
The invention relates to the field of magnetic telegraphones, and more specifically, to magnetic tape units employing one or more rotating heads which record and/or reproduce machine-convertible information while moving in transducing relationship with a stationary magnetic web or tape. This information being oriented as magnetic domains to form information tracks which extend generally traverse to the longitudinally tape length. In particular, the invention deals with servo-mechanical control of the motors in the tape unit so that a selected information track is brought into transducing relationship with the rotating heads.
2. Prior Art
Rotating head magnetic tape units are widely known. In one form, a generally cylindrical mandrel or drum includes a rotating head wheel which carries one or more read/write heads. The magnetic tape engages the mandrel at one point, makes a helical wrap about at least a portion of the mandrel, then exits the mandrel at a point which is both axially and circumferentially spaced from the entrance point. The angle of helical tape wrap can vary in accordance with design choice, but is usually between 180.degree. and 360.degree.. The head wheel rotates so as to sweep its magnetic heads traversely across the tape. The angle at which the head enters and exits the tape may vary, in accordance with design choice, from slightly less than 90.degree. to a small angle, such as 15.degree..
Another form of device is one wherein the head wheel is associated with a tape guiding structure which bends the tape traversely into an arcuate shape that conforms to the circumferential shape of the head wheel. In this device the tape travels in a generally straight line past the head wheel and is traversely bent by the associated guides as it enters the head wheel area.
The present invention finds utility with either aforementioned type of device and has been found particularly useful with helical wrap device.
The format of the magnetic media is essential for the proper operation of the above-described devices. Basically, the magnetic media has identification tracks, servo tracks and data tracks. The servo tracks are substantially parallel to the horizontal edge of the magnetic media; so that as the magnetic head makes a traverse sweep across the tape, either the servo tracks or the identification tracks are first encountered. Likewise, the data tracks are inclined to the servo tracks at an angle substantially equivalent to the angle at which the head enters and exits the tape. With this orientation, as the head sweeps traversely across the tape, data is transduced (i.e., read or write) from a selected data track.
There are two major problems associated in transducing data with the above devices.
One of the major problems encountered is that of establishing and maintaining accurate positional alignment between the path of the head wheel carrying the read/write head and the tape's data track. This is particularly true when the data track is written on one tape transport unit and later read by another tape unit. This problem is solved by U.S. Pat. No. 3,864,739 issued to Howard C. Jackson and U.S. Pat. No. 3,845,500 issued to Gary A. Hart. Both patents are assigned to International Business Machines Corporation, the assignee of the present invention.
The other major problem with which the present application is concerned is that of controlling the motor which drives the take-up spool of the tape system; so as to reduce overshoot and/or undershoot of the motor.
In order to step the tape from one data track to the next data track requires that the take-up spool be rotated. By energizing the take-up spool motor, the step function is performed. Once the motor is energized, the motor tends to hunt or ring (i.e., the motor tends to oscillate about its zero point before it comes to a final stop.) This hunting either tends to skew the tape in relationship with the head or aligning the incorrect data track with the head. Either situation, i.e., skewing or presenting the incorrect data track results in the system issuing servo errors or writing data in the wrong data track.
Two phenomenons are responsible for the damp oscillatory mode of the motor. The first phenomena is due to the gain of the system, while the second phenomena is due to the inertia of the load, i.e., the inertia of the media on the take-up spool.
It is well known in the prior art that the ringing of a motor driving a load can be controlled by changing the gain of the motor. In the so-called bang-bang servo mechanical positional control system, of the prior art, the driving arrangement (i.e., motor) is operated at maximum acceleration until the midpoint between an initial and selected positions is reached, and at maximum deceleration from the midpoint to the selected position. In theory, such a system will provide an optimum drive between positions with minimum oscillation about the selected position since the equal but opposite accelerations which switch at the midpoint position provided that zero velocity is reached at the selected position.
However, in practice, zero velocity is not always reached when the motor approaches the selected position. The residual velocity enables the motor to hunt or ring about the selected position. Due to hunting the bang-bang method of motor control is not satisfactory for several applications wherein precise motor control is required.
Moreover although the bang-bang systems function satisfactorily for their intended purposes, these systems are beset by several problems which render them inapplicable for this precision servomechanical position control system.
One of the problems which the bang-bang system faces is the ability to determine accurately the midpoint between the initial and selected position so that the acceleration/deceleration energy can be applied to the driving arrangement. The prior art system uses various sophisticated and expensive electronic circuitry for sensing the midpoint position. Due to the high cost for the electronic circuitry, the unit cost of the system tends to increase. The current trend is to minimize the cost of systems, and therefore the prior art devices are less acceptable.
Another problem with the prior art device is the constraint that the load had to be fixed, i.e., constant. With a constant load, the gain of the driving arrangement (e.g., an amplifier/motor combination) can be adjusted at the factory to meet optimum positioning of the load without overshoot or undershoot. In other words, once the constant inertia of a load is known, then the gain of the driving arrangement is adjusted so that the energization to the driving arrangement is such that the overshoot or undershoot (i.e., ringing) is minimized.
The prior art scheme works satisfactorily when the amplifier/motor combination is driving a constant load. However, there are several applications wherein the load changes constantly; i.e., the load is variable. For example, in the rotating head tape system previously discussed, the amount of tape on the take-up spool constantly changes. With the changes in tape, the inertia (load) of the take-up spool varies. It is, therefore, necessary to be able to control the motor when the load varies so as to minimize overshoot and/or undershoot. Since the prior art devices are restricted to fixed load conditions, they are not suitable to control the motor in a situation where the load varies.
Still another problem with the prior art device is that the gain of the driving arrangement is fixed. Generally, the gain of the prior art systems is controlled by a potentiometer (that is, a variable resistance). With the prior art devices, the potentiometer is adjusted so as to set the gain to a fixed value prior to delivery for shipment from the factory. Any attempt to change the gain from its previous setting requires the service of a highly skilled engineer or technician. This stems from the fact that changing the gain of the system will affect the overall operation of the device. In fact, there are several systems wherein the gain of the system is fixed permanently and cannot be varied. However, there are several instances wherein it is necessary to dynamically adjust the system gain so as to enhance the system performance. For example, in a situation wherein the frictional force in the system changes, inability to dynamically adjust the system gain will result in performance degradation.